In Ind. Eng. Chem. Res. 1991, 30, 1086-1092, Macromolecules 1992, 25, 883-886, J. Mol. Catal. A:Chem. 1998, 135, 121-132 and Rubber Chem. Technol. 2008, 81, 227-243 the Rhodium-based catalyst tris(triphenylphosphine)rhodium(I)chloride of formula (1) is disclosed for hydrogenation and hydrosilylation reactions of rubbers. However, high costs are associated with this catalyst which additionally requires the use of triphenylphosphine as co-catalyst. The catalyst degrades at 145° C.

In Chem. Comm. 1967, 305-306, Chem. Eur. J. 2010, 16, 12214-12220 and Tetrahedron Lett. 1966, 4871-4875 it is disclosed, that the complex tris(triphenylphosphine) hydrido ruthenium chloride of formula (2) can be used in a transfer hydrogenation for converting alkynes to alkenes. However, such catalyst does not efficiently hydrogenate nitrile rubbers and it is not selective for only olefins.

In J. Am. Chem. Soc. 1961, 83, 1262-1263, Chem. Eur. J. 2010, 16, 12214-12220, Am. Chem. Soc. 2010, 132, 16756-16758 and J. Mol. Catal. A:Chem. 2003, 206, 13-21 the catalyst of formula (3) as shown below is used as a transfer hydrogenation catalyst for alkynes to alkenes and for hydrogenation of amides to alcohols and amines under H2. However, such a catalyst is not selective for olefins and contains a CO group.

In Organometallics 2006, 25, 99-110, Dalton Trans. 2008, 2603-2614, Organometallics 2009, 28, 1758-1775, Inorg. Chim Acta. 2010, 363, 625-632 and Organometallics, 2010, 29, 5450-5455 the catalyst of formula (4) as shown below is prepared from RuHCl(CO)(AsPh3)3 and IMes2. Such preparation method, however, is not favorable due to the presence of AsPh3. The catalyst further contains a CO group. Such catalyst is described for transfer hydrogenation of aromatic ketones with alcohols. It also hydrogenates olefins and ketones using H2, however, it is not selective for olefins.

According to Organometallics 2004, 23, 86-94, the catalyst of formula (5) as shown below can be prepared from RuHCl(PPh3)3 and two equivalents of SIMes2 with the formation of SIMes2HCl as a by-product. However, no hydrogenation data is reported. It is not possible to displace PPh3 with SIMes2 without CH activation of the methyl groups.

In Dalton Trans., 2013, 42, 2362-2365 the catalyst of the formula (6) as shown below is prepared from [(p-cymene)RuCl2]2 and a triazolium salt with following treatment with K2CO3 in THF under reflux. However, no hydrogenation data is reported. A specific use for these 1,2,3-triazol-5-ylidene (tzNHC) ruthenium complexes is not disclosed in the paper.

Similar complexes with different metals and different ligands have been disclosed in the past.
In Dalton Trans., 2012, 41, 13074-13080 the catalyst of the formula (7) as shown below is prepared from IrCl2(Cp*)(trz) (trz=triazolylidene) and sodium acetate under nitrogen. This iridium complex displays high potential as a water oxidation catalyst. The use as a hydrogenation catalyst is not disclosed. The use of other metals like ruthenium is also not suggested.

In Angew. Chem. Int. Ed. 2010, 49, 9765-9768 the catalyst of the formula (8) as shown below is prepared from [(Cp*IrCl2)2] (Cp*=C5Me5), Ag2O and a pyridinium-functionalized triazolium salt. Iridium(III) cyclopentadienyl complexes are described to exhibit excellent activity in electrochemical induced water oxidation. However, no hydrogenation data is reported. Ruthenium-based complexes are neither disclosed nor suggested.

In Organometallics 2011, 30, 1689-1694 the catalyst of the formula (9) as shown below is prepared from Pd(OAc)2 and a trans mono nuclear silver carbene complex [(Tz)2Ag] (Tz=1,4-diphenyl-3-methyl-1,2,3-triazol-5-ylidene). However, no hydrogenation data is reported.

Summing up various complexes, which are used as catalysts, are already available for hydrogenation reactions. However, many of them contain unfavorable or expensive ligands, like PPh3, form gels, are difficult to prepare, are not sufficiently active and/or selective.
Therefore, it was the object of the present invention to provide an inexpensive, thermally robust, gel formation inhibiting and olefin selective novel catalyst for hydrogenation reactions, particularly for hydrogenating polymers, and even more particularly for hydrogenating nitrile rubbers.